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在Linux下开发应用程序可以调用两种接口来实现，一种是直接调用系统调用接口，另一种是调用库函数来实现。
1、什么是系统调用？系统调用（System Call）是操作系统提供的服务，是应用程序与内核通信的接口。Linux提">
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                <h2 id="应用程序开发方式"><a href="#应用程序开发方式" class="headerlink" title="应用程序开发方式"></a>应用程序开发方式</h2><p><img src="https://s1.ax1x.com/2020/07/04/Nz5OiT.png" alt></p>
<p>在Linux下开发应用程序可以调用两种接口来实现，一种是直接调用<code>系统调用</code>接口，另一种是调用<code>库函数</code>来实现。</p>
<h3 id="1、什么是系统调用？"><a href="#1、什么是系统调用？" class="headerlink" title="1、什么是系统调用？"></a>1、什么是系统调用？</h3><p>系统调用（<code>System Call</code>）是操作系统提供的服务，是应用程序与内核通信的接口。Linux提供的系统调用包含的内容有：文件操作、进程控制、系统控制、内存管理、网络管理、socket套接字、进程间通信、用户管理等。</p>
<p><img src="https://s1.ax1x.com/2020/07/04/Nz5zQJ.png" alt><br><img src="https://s1.ax1x.com/2020/07/04/Nz5vz4.png" alt><br><img src="https://s1.ax1x.com/2020/07/04/Nz5jWF.png" alt><br><img src="https://s1.ax1x.com/2020/07/04/Nz5XJU.png" alt></p>
<p>相对于普通的函数调用来说， 系统调用的<code>性能消耗</code>也是<code>巨大</code>的。 所以在追求极致性能的程序中， 都在尽力避免系统调用。</p>
<h3 id="2、C库函数"><a href="#2、C库函数" class="headerlink" title="2、C库函数"></a>2、C库函数</h3><p>C标准库头文件如下：</p>
<p><img src="https://s1.ax1x.com/2020/07/04/NzIkFK.jpg" alt></p>
<p>Linux环境下， 使用的C库一般都是glibc， 它封装了几乎所有的系统调用， 代码中使用的“系统调用”， 实际上就是调用C库中的函数。 因为C库函数通过系统调用来实现，库函数对上层提供了C库接口。比如文件操作：</p>
<p><img src="https://s1.ax1x.com/2020/07/04/NzIpLR.png" alt></p>
<h2 id="实例：文件操作"><a href="#实例：文件操作" class="headerlink" title="实例：文件操作"></a>实例：文件操作</h2><p>Linux下文件操作应用程序结构如下：</p>
<p><img src="https://s1.ax1x.com/2020/07/04/NzICe1.png" alt></p>
<p>Linux下，“一切皆文件”。为了使不同的文件系统共存， Linux 内核在用户层与具体文件系统之前增加了<code>虚拟文件系统（VFS）</code>中间层，它对复杂的系统进行抽象化，对用户提供了统一的文件操作接口。 </p>
<h3 id="1、文件操作：系统调用"><a href="#1、文件操作：系统调用" class="headerlink" title="1、文件操作：系统调用"></a>1、文件操作：系统调用</h3><p><strong>（1）实操</strong></p>
<p><strong>代码：</strong></p>
<pre class=" language-c"><code class="language-c"><span class="token comment" spellcheck="true">/* 文件操作：系统调用 */</span>
<span class="token macro property">#<span class="token directive keyword">include</span> <span class="token string">&lt;sys/stat.h></span></span>
<span class="token macro property">#<span class="token directive keyword">include</span> <span class="token string">&lt;unistd.h></span></span>
<span class="token macro property">#<span class="token directive keyword">include</span> <span class="token string">&lt;fcntl.h></span></span>
<span class="token macro property">#<span class="token directive keyword">include</span> <span class="token string">&lt;stdio.h></span></span>
<span class="token macro property">#<span class="token directive keyword">include</span> <span class="token string">&lt;stdlib.h></span></span>

<span class="token keyword">int</span> <span class="token function">main</span><span class="token punctuation">(</span><span class="token keyword">void</span><span class="token punctuation">)</span>
<span class="token punctuation">{</span>
    <span class="token keyword">int</span> fd<span class="token punctuation">;</span>                   <span class="token comment" spellcheck="true">/* 文件描述符 */</span>
    <span class="token keyword">char</span> rd_buf<span class="token punctuation">[</span><span class="token number">50</span><span class="token punctuation">]</span> <span class="token operator">=</span> <span class="token punctuation">{</span><span class="token number">0</span><span class="token punctuation">}</span><span class="token punctuation">;</span> <span class="token comment" spellcheck="true">/* 读数据buf  */</span> 

    <span class="token comment" spellcheck="true">/* 以读写的方式打开文件demo0_test，若文件不存在则创建文件 */</span>
    fd <span class="token operator">=</span> <span class="token function">open</span><span class="token punctuation">(</span><span class="token string">"demo0_test"</span><span class="token punctuation">,</span> O_RDWR<span class="token operator">|</span>O_CREAT<span class="token operator">|</span>O_TRUNC<span class="token punctuation">,</span> S_IRWXU<span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token keyword">if</span> <span class="token punctuation">(</span><span class="token constant">NULL</span> <span class="token operator">==</span> fd<span class="token punctuation">)</span>
    <span class="token punctuation">{</span>
        <span class="token function">printf</span><span class="token punctuation">(</span><span class="token string">"Open file error!\n"</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
        <span class="token keyword">return</span> <span class="token number">0</span><span class="token punctuation">;</span>
    <span class="token punctuation">}</span>

    <span class="token comment" spellcheck="true">/* 往文件中写数据 */</span>
    <span class="token function">write</span><span class="token punctuation">(</span>fd<span class="token punctuation">,</span> <span class="token string">"helllo world\n"</span><span class="token punctuation">,</span> <span class="token function">strlen</span><span class="token punctuation">(</span><span class="token string">"helllo world\n"</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>

    <span class="token comment" spellcheck="true">/* 移动文件指针至文件开头 */</span>
    <span class="token function">lseek</span><span class="token punctuation">(</span>fd<span class="token punctuation">,</span> <span class="token number">0</span><span class="token punctuation">,</span> SEEK_SET<span class="token punctuation">)</span><span class="token punctuation">;</span>

    <span class="token comment" spellcheck="true">/* 读出文件数据并打印 */</span>
    <span class="token function">read</span><span class="token punctuation">(</span>fd<span class="token punctuation">,</span> rd_buf<span class="token punctuation">,</span> <span class="token number">50</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token function">printf</span><span class="token punctuation">(</span><span class="token string">"Read data: %s\n"</span><span class="token punctuation">,</span> rd_buf<span class="token punctuation">)</span><span class="token punctuation">;</span>

    <span class="token comment" spellcheck="true">/* 关闭文件 */</span>
    <span class="token function">close</span><span class="token punctuation">(</span>fd<span class="token punctuation">)</span><span class="token punctuation">;</span>

    <span class="token keyword">return</span> <span class="token number">0</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span></code></pre>
<p><strong>运行结果：</strong></p>
<p><img src="https://s1.ax1x.com/2020/07/04/NzIPdx.png" alt></p>
<p><strong>（2）接口说明</strong></p>
<p><strong>open函数：</strong></p>
<p><img src="https://s1.ax1x.com/2020/07/04/NzIio6.png" alt></p>
<p>• pathname：要打开或创建的文件名；</p>
<p>• flag：指定文件的打开方式，具体有以下参数，见下表 flag 参数值。 </p>
<p><img src="https://s1.ax1x.com/2020/07/04/NzIZSe.png" alt></p>
<p>• mode：当 open 函数的 flag 值设置为 O_CREAT 时，必须使用 mode 参数来设置文件与用户相关的<br>权限。 mode 可用的权限如下表所示，表中各个参数可使用” | “来组合。</p>
<p><img src="https://s1.ax1x.com/2020/07/04/NzImyd.png" alt></p>
<p><strong>read函数：</strong> </p>
<p><img src="https://s1.ax1x.com/2020/07/04/NzIAJO.png" alt></p>
<p>• fd：文件对应的文件描述符 ；</p>
<p>• buf：指向数据缓冲区的指针； </p>
<p>• count：读取多少个字节的数据。 </p>
<p><strong>write函数：</strong></p>
<p><img src="https://s1.ax1x.com/2020/07/04/NzIEWD.png" alt></p>
<p>• fd：文件对应的文件描述符；</p>
<p>• buf：指向数据缓冲区的指针；</p>
<p>• count：往文件中写入多少个字节。 </p>
<p><strong>close函数：</strong></p>
<p><img src="https://s1.ax1x.com/2020/07/04/NzIeQH.png" alt></p>
<p>关闭 fd 文件描述符对应的文件。</p>
<p><strong>lseek 函数：</strong></p>
<p><img src="https://s1.ax1x.com/2020/07/04/NzIKeI.png" alt></p>
<p>lseek 函数可以用与设置文件指针的位置，并返回文件指针相对于文件头的位置。 其中的 offset 参数用于指定位置， whence 参数则定义了 offset 的意义， whence 的可取值如下：<br>• SEEK_SET： offset 是一个绝对位置。</p>
<p>• SEEK_END： offset 是以文件尾为参考点的相对位置。</p>
<p>• SEEK_CUR： offset 是以当前位置为参考点的相对位置。 </p>
<h3 id="2、文件操作：C库函数"><a href="#2、文件操作：C库函数" class="headerlink" title="2、文件操作：C库函数"></a>2、文件操作：C库函数</h3><p><strong>（1）实操</strong></p>
<p><strong>代码：</strong></p>
<pre class=" language-c"><code class="language-c"><span class="token comment" spellcheck="true">/* 文件操作：c库函数 */</span>
<span class="token macro property">#<span class="token directive keyword">include</span> <span class="token string">&lt;stdio.h></span></span>
<span class="token macro property">#<span class="token directive keyword">include</span> <span class="token string">&lt;stdlib.h></span></span>

<span class="token keyword">int</span> <span class="token function">main</span><span class="token punctuation">(</span><span class="token keyword">void</span><span class="token punctuation">)</span>
<span class="token punctuation">{</span>
    FILE <span class="token operator">*</span>fp<span class="token punctuation">;</span>               <span class="token comment" spellcheck="true">/* 文件描述符 */</span>
    <span class="token keyword">char</span> rd_buf<span class="token punctuation">[</span><span class="token number">50</span><span class="token punctuation">]</span> <span class="token operator">=</span> <span class="token punctuation">{</span><span class="token number">0</span><span class="token punctuation">}</span><span class="token punctuation">;</span> <span class="token comment" spellcheck="true">/* 读数据buf  */</span> 

    <span class="token comment" spellcheck="true">/* 以读写的方式打开文件demo1_test，若文件不存在则创建文件 */</span>
    fp <span class="token operator">=</span> <span class="token function">fopen</span><span class="token punctuation">(</span><span class="token string">"demo1_test"</span><span class="token punctuation">,</span> <span class="token string">"w+"</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token keyword">if</span> <span class="token punctuation">(</span><span class="token constant">NULL</span> <span class="token operator">==</span> fp<span class="token punctuation">)</span>
    <span class="token punctuation">{</span>
        <span class="token function">printf</span><span class="token punctuation">(</span><span class="token string">"Open file error!\n"</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
        <span class="token keyword">return</span> <span class="token number">0</span><span class="token punctuation">;</span>
    <span class="token punctuation">}</span>

    <span class="token comment" spellcheck="true">/* 往文件中写数据 */</span>
    <span class="token function">fwrite</span><span class="token punctuation">(</span><span class="token string">"hello world\n"</span><span class="token punctuation">,</span> <span class="token number">1</span><span class="token punctuation">,</span> <span class="token function">strlen</span><span class="token punctuation">(</span><span class="token string">"helllo world\n"</span><span class="token punctuation">)</span><span class="token punctuation">,</span> fp<span class="token punctuation">)</span><span class="token punctuation">;</span>

    <span class="token comment" spellcheck="true">/* 同步缓冲区：把缓冲区数据立即写入文件 */</span>
    <span class="token function">fflush</span><span class="token punctuation">(</span>fp<span class="token punctuation">)</span><span class="token punctuation">;</span>

    <span class="token comment" spellcheck="true">/* 移动文件指针至文件开头 */</span>
    <span class="token function">fseek</span><span class="token punctuation">(</span>fp<span class="token punctuation">,</span> <span class="token number">0</span><span class="token punctuation">,</span> SEEK_SET<span class="token punctuation">)</span><span class="token punctuation">;</span>

    <span class="token comment" spellcheck="true">/* 读出文件数据并打印 */</span>
    <span class="token function">fread</span><span class="token punctuation">(</span>rd_buf<span class="token punctuation">,</span> <span class="token number">50</span><span class="token punctuation">,</span> <span class="token number">1</span><span class="token punctuation">,</span> fp<span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token function">printf</span><span class="token punctuation">(</span><span class="token string">"Read data: %s\n"</span><span class="token punctuation">,</span> rd_buf<span class="token punctuation">)</span><span class="token punctuation">;</span>

    <span class="token comment" spellcheck="true">/* 关闭文件 */</span>
    <span class="token function">fclose</span><span class="token punctuation">(</span>fp<span class="token punctuation">)</span><span class="token punctuation">;</span>

    <span class="token keyword">return</span> <span class="token number">0</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span></code></pre>
<p><strong>运行结果：</strong></p>
<p><img src="https://s1.ax1x.com/2020/07/04/NzIMwt.png" alt></p>
<p><strong>（2）接口说明</strong></p>
<p><strong>fopen函数：</strong></p>
<p><img src="https://s1.ax1x.com/2020/07/04/NzI1Ff.png" alt></p>
<p>• pathname 参数用于指定要打开或创建的文件名。</p>
<p>• mode 参数用于指定文件的打开方式，注意该参数是一个字符串，输入时需要带双引号：</p>
<p>•“r”：以只读方式打开，文件指针位于文件的开头。</p>
<p>•“r+”：以读和写的方式打开，文件指针位于文件的开头。</p>
<p>•“w”：以写的方式打开，不管原文件是否有内容都把原内容清空掉，文件指针位于文件的开头。</p>
<p>•“w+”：同上，不过当文件不存在时，前面的” w”模式会返回错误，而此处的” w+”则会创建新文件。</p>
<p>•“a”：以追加内容的方式打开，若文件不存在会创建新文件，文件指针位于文件的末尾。与” w+”的区<br>别是它不会清空原文件的内容而是追加。</p>
<p>•“a+”：以读和追加的方式打开，其它同上。</p>
<p>• fopen 的返回值是 FILE 类型的文件文件流，当它的值不为 NULL 时表示正常，后续的 fread、 fwrite<br>等函数可通过文件流访问对应的文件。 </p>
<p>fopen 的 mode 与 open 的 flags 参数关系：</p>
<p><img src="https://s1.ax1x.com/2020/07/04/NzIJSg.png" alt></p>
<p><strong>fread 函数：</strong></p>
<p><img src="https://s1.ax1x.com/2020/07/04/NzIQTP.png" alt></p>
<p>stream 是使用 fopen 打开的文件流， fread 通过它指定要访问的文件，它从该文件中读取 nmemb 项数据，每项的大小为 size，读取到的数据会被存储在 ptr 指向的数组中。 fread 的返回值为成功读取的项数（项的单位为 size）。 </p>
<p><strong>fwrite 函数：</strong></p>
<p><img src="https://s1.ax1x.com/2020/07/04/NzI3Y8.png" alt></p>
<p>它的操作与 fread 相反，把 ptr 数组中的内容写入到 stream 文件流，写入的项数为 nmemb，每项大小为size，返回值为成功写入的项数（项的单位为 size）。 </p>
<p><strong>fclose 函数：</strong> </p>
<p><img src="https://s1.ax1x.com/2020/07/04/NzI8fS.png" alt></p>
<p>fclose 库函数用于关闭指定的文件流，关闭时它会把尚未写到文件的内容都写出。因为标准库会对数据进行缓冲，所以需要使用 fclose 来确保数据被写出。 </p>
<p><strong>fflush 函数：</strong> </p>
<p><img src="https://s1.ax1x.com/2020/07/04/NzIYlQ.png" alt></p>
<p>fflush 函数用于把尚未写到文件的内容立即写出。常用于确保前面操作的数据被写入到磁盘上。 fclose 函数本身也包含了 fflush 的操作。</p>
<p><strong>fseek 函数：</strong></p>
<p><img src="https://s1.ax1x.com/2020/07/04/NzItyj.png" alt></p>
<p>其中的 offset 参数用于指定位置， whence 参数则定义了 offset 的意义， whence 的可取值如下：<br>• SEEK_SET： offset 是一个绝对位置。</p>
<p>• SEEK_END： offset 是以文件尾为参考点的相对位置。</p>
<p>• SEEK_CUR： offset 是以当前位置为参考点的相对位置。 </p>
<p>以上就是本次的分享，如有错误，欢迎指出！</p>
<p><strong>相关参考书籍：</strong></p>
<p>《Linux环境编程：从应用到内核》</p>
<p>《野火Linux开发实战指南》</p>
<p>《嵌入式Linux上的C语言编程实践》</p>
<hr>
<p>我的个人博客：<a href="https://www.lizhengnian.cn/">https://www.lizhengnian.cn/</a></p>
<p>我的微信公众号：嵌入式大杂烩</p>
<p>我的CSDN博客：<a href="https://blog.csdn.net/zhengnianli" target="_blank" rel="noopener">https://blog.csdn.net/zhengnianli</a></p>
<p><img src="https://s1.ax1x.com/2020/06/28/NgPwRK.png" alt></p>

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